The present invention relates to a variable-venturi-type carburetor in an internal combustion engine, especially to a carburetor having a slow speed system.
Variable-venturi-type carburetors having a slow speed system are known. Such a carburetor has a suction piston movable in the intake passage in response to a change in the amount of air flowing within the intake passage for varying the cross-sectional flow area of the intake passage, and has a main fuel metering system comprising a metering needle fixed onto the suction piston and a metering jet arranged in one end of a main fuel passage which extends along the longitudinal axis of the needle to allow the needle to enter therein. Further, this type carburetor has a slow speed system comprising a slow fuel passage, an inlet of which is open to the main fuel passage and an outlet of which is open to the intake passage of the carburetor.
For obtaining an economical fuel-consuming engine, especially during engine idling, it is necessary that the inner diameter of the slow fuel passage be relatively small. However, the atmospheric temperature in the area of the carburetor is high, and the temperature of the carburetor increases when the engine is caused to idle after being driven at a high load. The fuel is vaporised in the main fuel passage during such conditions. If fuel vapor is introduced into the relatively small slow fuel passage during engine idling, the air-fuel ratio varies in step pulses. This causes the engine revolution to be unstable and the engine may stall.